1. Field of the Invention
The invention relates to a control mechanism in a brush-honing machine to compensate for the wear of the brushes.
2. Prior Art
Brush-honing is a process to improve honed surfaces of borings or the like as described, for example, in European patent application EP-A1-0 247 572 (published Dec. 2, 1987). Actually, after honing, the honed surfaces are further treated with brushes under certain conditions to improve the quality of the surfaces by reducing burrs and grate-like sheet metal particles, which are generated and not removed in the preceding honing process.
Further, such brush-honing treatment may be used to provide the end treatment of borings in workpieces of certain aluminum alloys, especially silica containing aluminum alloys like AlSi.sub.17 Cu.sub.4 MG, in which relatively hard silicon crystals are embedded in a considerably softer aluminum matrix. If a boring in a workpiece of such a material, e.g. in a cylinder block for a combustion engine, is treated by brush-honing, the soft aluminum matrix will be somewhat moved around the harder silicon crystals, which will remain unaffected by this operation, such that thereafter the harder crystals project a little over the surface of the aluminum matrix . The projecting crystals will serve as bearing surfaces for any corresponding and/or cooperating other metal part, e.g. for the piston in an engine block. At the same time, the softer portions, of which some material has been removed, will serve as pockets for lubricating liquids. This treatment is applied especially to high-quality aluminum cylinder blocks for automobile engines, which, before such brush-honing method had been developed, were treated by a complicated etching process.
In connection with such a brush-honing operation, the brushes basically are applied in the same manner as honing tools are applied in the honing process, i.e. they are introduced into the boring and, while the free ends of the brushes or bristles respectively are pressed with some predetermined tension radially against the interior wall of the boring, they are rotated and at the same time moved upwardly and downwardly in the axial direction of the boring. The brushing then essentially follows the cross pattern, which has been produced in the honing process. Rotation may also be reversed when used in connection with the mentioned treatment of boring silicon containing alloys.
In order to maintain a certain predetermined tension of the free ends of the bristles of the brushes against the interior wall of the boring despite their wear, the tools, which carry the brush elements, can expand in the radial direction. The tension of the brush or bristles is usually measured by the amount with which the outer diameter of the brush is larger than the diameter of the boring of the workpiece. However, the counter pressure exerted by the wall of the boring on the bristles of the brush and on the tool is not strong enough to keep the brush at a position in which the tension of the brush or the bristles is exactly determined.
In other words, the adaption of conventional honing-machines for such brush-honing as are described in European patent application EP-A1-0 247 572, is rather difficult, since brush-honing can not rely on an exact position of a surface-to-surface-contact of the tool against the workpiece to define a predetermined tension of the brush or bristles respectively against the surface of the workpiece. Further, the rotational speed of the tool (up to 400 rpm) is much higher than with conventional honing, while the pressure of the tool against the workpiece surface is much less.